We are testing the hypothesis that Streptococcus mutans is responsible for a measurable amount of decay in humans. Rampant caries individuals, i.e. individuals with greater than or equal to 10 carious teeth, have high levels of Strep mutans in their plaque. We are making a bacteriological diagnosis of Strep mutans infection and treating these individuals topically with either 5 per cent Kanamycin or 1.23 per cent NaF in a pleasant tasting gel. The subjects are treated twice a day for a one week period on two occasions, i.e. prior to the placement of dental restorations and immediately after the placement of the restorations. Fissure, and proximal plaque samples and saliva are cultured before and after each treatment period and at the recall periods for total viable counts, Strep mutans and strep sanguis counts. A placebo group is included. Subjects are randomly assigned to the treatment groups. The clinical scoring of caries and the bacteriological analysis are performed in an double blind procedure. An I.N.D. for chlorhexidine gluconate has been approved by the F.D.A. and soon chlorhexidine will be used in these studies. In separate longitudinal studies we are monitoring the bacterial flora of caries free occlusal fissures for their S. mutans, S. sanguis and lactobacilli content. Thus far over 200 surfaces have been sampled many times. BIBLIOGRAPHIC REFERENCES: Loesche, W.J. Microbial agents for the treatment of dental caries. J. Amer. Soc. Preventive Dent., 5:17-25, 1975. Loesche, W.J. and E. Grenier, Detection of Streptococcus mutans in plaque samples by the direct fluorescent antibody test. J. Dent. Res., 55. Supplement A, 1976.